The principal components of a prior art KrF excimer laser chambers are shown in FIG. 1. This chamber is a part of a laser system used as a light source for integrated circuit lithography. These components include a chamber housing 2. The housing contains two electrodes 84 and 83 each about 50 cm long and spaced apart by about 20 mm, a blower 4 for circulating a laser gas between the electrodes at velocities fast enough to clear (from a discharge region between the two electrodes) debris from one pulse prior to the next succeeding pulse at a pulse repetition rate in the range of 1000 Hz or greater, and a water cooled finned heat exchanger 6 for removing heat added to the laser gas by the fan and by electric discharges between the electrodes. The word xe2x80x9cdebrisxe2x80x9d is used here to define any physical condition of the gas after a laser pulse which is different from the condition of the gas prior to said pulse. The chamber may also include baffles and vanes for improving the aerodynamic geometry of the chamber. The laser gas is comprised of a mixture of about 0.1 percent fluorine, about 1.0 percent krypton and the rest neon. Each pulse is produced by applying a very high voltage potential across the electrodes with a pulse power supply 8 which causes a discharge between the electrodes lasting about 30 nanoseconds to produce a gain region about 20 mm high, 3 mm wide and 500 mm long. The discharge deposits about 2.5 J of energy into the gain region. As shown in FIG. 2, lasing is produced in a resonant cavity, defined by an output coupler 2A and a grating based line narrowing unit (called a line narrowing package or LNP, shown disproportionately large) 2B comprising a three prism beam expander, a tuning mirror and a grating disposed in a Littrow configuration. The energy of the output pulse 3 in this prior art KrF lithography laser is typically about 10 mJ.
There are many industrial applications of electric discharge lasers. One important use is as the light source for integrated circuit lithography machines. One such laser light source is the line narrowed KrF laser described above which produces a narrow band pulsed ultraviolet light beam at about 248 nm. These lasers typically operate in bursts of pulses at pulse rates of about 1000 to 4000 Hz. Each burst consists of a number of pulses, for example, about 80 pulses, one burst illuminating a single die section on a wafer with the bursts separated by off times of a fraction of a second while the lithography machine shifts the illumination between die sections. There is another off time of a few seconds when a new wafer is loaded. Therefore, in production, for example, a 2000 Hz, KrF excimer laser may operate at a duty factor of about 30 percent. The operation is 24 hours per day, seven days per week, 52 weeks per year. A laser operating at 2000 Hz xe2x80x9caround the clockxe2x80x9d at a 30 percent duty factor will accumulate more than 1.5 billion pulses per month. Any disruption of production can be extremely expensive. For these reasons, prior art excimer lasers designed for the lithography industry are modular so that maintenance down time is minimized.
Maintaining high quality of the laser beam produced by these lasers is very important because the lithography systems in which these laser light sources are used are currently required to produce integrated circuits with features smaller than 0.25 microns and feature sizes get smaller each year. As a result the specifications placed on the laser beam limit the variation in individual pulse energy, the variation of the integrated energy of series of pulses, the variation of the laser wavelength and the magnitude of the bandwidth of the laser beam.
Typical operation of electric discharge lasers such as that depicted in FIG. 1 causes electrode erosion. Erosion of these electrodes affects the shape of the discharge which in turn affects the quality of the output beam as well as the laser efficiency. Electrode designs have been proposed which are intended to minimize the effects of erosion by providing on the electrode a protruding part having the same width as the discharge. Some examples are described in Japanese Patent No. 2631607. These designs, however, produce adverse effects on gas flow. In these gas discharge lasers, it is very important that virtually all effects of each pulse be blown out of the discharge region prior to the next pulse.
Another discharge laser, very similar to the KrF laser is the argon fluorine (ArF) laser. In this laser the gas is a mixture primarily of argon fluorine and neon, and the wavelength of the output beam is in the range of about 193 nm. These ArF lasers are just now being used to a significant extend for integrated circuit fabrication, but the use of these lasers is expected to grow rapidly. Still another discharge laser expected to be used extensively in the future for integrated circuit fabrication is the F2 laser where the gas is F2 and a buffer gas could be neon or helium or a combination of neon and helium.
What is needed is a gas discharge laser having electrodes which do not adversely affect gas flow and can withstand many billions of pulses without eroding sufficiently to adversely affect the laser beam quality.
The present invention provides a gas discharge laser having an elongated cathode and an elongated anode with a porous insulating layer covering the anode discharge surface. A pulse power system provides electrical pulses at rates of at least 1 KHz. A blower circulates laser gas between the electrodes at speeds of at least 5 m/s and a heat exchanger is provided to remove heat produced by the blower and the discharges. In preferred embodiments at least a portion of the anode is comprised of lead, and fluorine ion sputtering of the anode surface creates the insulating layer (over the discharge surface of the anode) comprised in large part of lead fluoride. In a particular preferred embodiment the anode is fabricated in two parts, a first part having the general shape of a prior art anode with a trench shaped cavity at the top. This part is comprised of brass comprised of less than 1% lead. A second part comprised of brass having a lead content of greater than 3% is soldered into the trench and protrudes above the surface by about 0.2 millimeter. When the anode is installed in the laser and is subjected to pulse discharges in a fluorine containing laser gas environment an insulating layer, comprising porous lead fluoride, forms on the surface of the second part protecting it from significant erosion, Applicants"" computer electric field models have shown that the insulating layer does not significantly affect the electric field between the cathode and the anode. Since the first part does not contain lead, no significant insulating layer forms on it, but the electric field distribution prevents any significant portion of the discharges from being attracted to the surface of the second part. To the extent discharges do occur on the first part, erosion will occur at the discharge sites reducing the height of the anode in the region of the discharge which has the effect of reducing the discharge from the first part. About 50,000 small holes develop in the insulating layer on the second part which permit electrons to flow freely to and from the metal surface of the anode. However, fluorine ion sputtering on the metal surface of the anode is substantially limited after to insulating layer develops. Applicants believe that the reduction in fluorine ion sputtering results from a reduced number of fluorine ions reaching the metal surface and a reduction in energy of the ions that do reach the metal surface.
Applicants"" tests have shown that a porous insulating layer that covers substantially all of the discharge surface of the anode does not significantly interfere with the electric field between the electrodes and does not significantly affect the shape of the discharge. (Adverse effects can result, however, if a substantial portion of the discharge region is covered and a substantial portion is not covered. In this case, the discharges tend to accumulate at the uncovered locations causing substantial erosion at those locations and severely distorting the uniformity of the discharge.) Thus, best performance results if the discharge surface is fully covered by the porous insulating layer or not covered by the layer at all. However, in the completely uncovered situation there is long term erosion whereas in the completely covered situation, we have very good performance and also minimum erosion over billions of pulses.